XSM Radiation Hardness Studies

1
XSM - A FINNISH INSTRUMENT FOR THE STUDIES OF
THE X-RAY SUN
HESA
J. Huovelin2, J. Laukkanen1, K. Hämäläinen1, L.
Alha2, P. Muhli2, O. Vilhu2, H. Andersson3, T.
Andersson3, V. Lämsä3, S. Nenonen3, H. Sipilä3,
I. Taylor4, M. Murray4, and B. Foing5 1 Division
of X-ray Physics, Department of Physical
Sciences, P.O.Box 64, FIN-00014 University of
Helsinki, Helsinki, Finland 2 Observatory,
Department of Astronomy, P.O.Box 14, FIN-00014
University of Helsinki, Helsinki, Finland 3
Metorex Oy, Nihtisillankuja 5, FIN-02630 Espoo,
Finland 4 Metorex Inc., Ewing, New Jersey,
USA 5 ESA, ESTEC/SCI-SO, Noordwijk,
Netherlands E-Mail Juhani.Huovelin_at_Helsinki.Fi,
Jarkko.Laukkanen_at_Helsinki.Fi
Introduction The continuous solar energy
production is of paramount importance to the life
on the Earth. A great majority of the total
energy output of the Sun is confined to the
wavelengths visible to the unaided eye. But the
Sun does also have a significant energy output
outside the region of the visible light.
Depending on the energy range and the phase of
the solar activity cycle, our view of the Sun can
be very different (Fig. 1.).
InfraRed (He-I) Visible Light
Visible Light (H-I) Visible Light
(Coronal H-I) Near UltraViolet
(He-II) UltraViolet (Fe-XV)
UltraViolet (Fe-IX/X) Soft
X-rays Fig. 2. Few views of the Sun, taken at
different energy regions of the electromagnetic
spectrum just before the solar outburst of April
2.-3., 2001. The phase of the Sun is identical in
every picture. Figures marked with an asterisk
() are artificially coloured. The figures are
courtesy of ESA/SOHO or ISAS/Yohkoh. The
details we see in the figures (Fig. 1.) reveal a
great deal of the physical processes at work in
the Sun. Especially, the figures taken in the
extreme ultraviolet and the soft X-ray ranges
show the highly dynamic nature of the Sun. The
struc-tures seen are not restricted only to the
surface of the Sun, they do extend far outside
the surface. Occasionally, the Sun also releases
huge outbursts of matter and radiation (Fig. 2.),
called flares and coronal mass ejections. Yet,
the exact nature of the physical processes behind
these events is largely unknown. F
ig. 2. The outburst of April 2.-3. 2001, as seen
by the imager onboard the SOHO satellite. The
figure is courtesy of ESA/SOHO.
Solar Flares Solar flares (Fig. 2.) may
release energies that are typically several
orders of magnitude beyond the annual energy
consumption of the whole human population. Thus,
these events will have an effect to the
space-weather conditions within the near-Earth
space, and possibly even to the life on the
Earth. The most familiar effect may be the
frequent occurrence of the northern lights
(Aurora Borealis). The exact mechanism behind
the Suns eruptive behavior is still somewhat
unknown. We already know that the coronal mass
ejections do consist mainly of highly ionized
gaseous matter which becomes thrown into the
interplanetary space at speeds as high as
thousands of km/s. From the locations the
eruptions originate, it is known that they are
connected to the active sunspots. Additionally,
from the structure of the loops seen alongside
many smaller flares it is believed that the
complex magnetic field of the Sun is the driving
force of these eruptions. ESA SMART-1 The
European Space Agency (ESA) will launch a small
technology satellite called SMART-1 (Fig. 3.) to
the Moon in the end of February 2003. The primary
goal of the mission is to test the applicability
of the novel ion-propulsion engine in maneuvering
complex satellite orbits within the solar system.
Additionally, SMART-1 will carry a small
scientific payload, which is intended for
geophysical and geochemical studies of the lunar
surface. Fig. 3. SMART-1, the
technology satellite by ESA, is the European
return to the Moon. The figure is courtesy of
ESA. The X-ray Solar Monitor (XSM) Among
the instruments onboard the SMART-1 is an imaging
X-ray spectrometer D-CIXS, intended for the
mapping of the elemental content of the surface
of the Moon by acquiring the lunar fluorescence
spectrum excited by the solar X-rays. The content
can be extracted from the fluorescence spectrum
only if the incident spectrum (solar X-rays) is
known accurately. This highly variable
information is provided by the XSM (Fig. 4.). The
XSM will also carry an independent solar science
program in monitoring the solar soft X-ray
spectrum for over a year. As the radiative output
of the Sun in this energy range is highly
sensitive to the dynamic conditions on the Sun,
the information gained by studying these spectra
will be very important to our understanding of
the Sun. Fig. 4. The X-ray Solar
Monitor (XSM) was designed and made in Finland.
The figure is courtesy of Metorex Oy.
The XSM unit is based on a modern detector
concept A Si-PIN diode (500mm thick, 1.5 mm2
area, 105? field-of-view) is acting as the X-ray
detector. It is cooled with a Peltier element to
an operational temperature of -20?C, providing an
energy resolution of 250 eV at the incident
energy of 5.9 keV (beginning of life, estimated
to degrade to 350 eV at the end of the 18-month
mission). The working specification for the X-ray
energy range of the XSM, 1 - 20 keV (Fig. 5.),
matches very nicely with the X-ray energy range
of highest interest to the science of solar
flares. Fig. 5. The effective area
of the XSM Si-PIN X-ray detector. Science with
the XSM The spectral range of the XSM is very
sensitive to solar flare activity. The data on
the evolution of a large number of flares
acquired during the 18-month SMART-1 mission will
yield a fairly large database for the studies in
flare physics. Yet, the XSM is still sensitive
enough to see the quiescent solar X-ray
spectrum. The time resolution of the spectra
(16s) also enables to follow the variations in
the mean coronal temperature and non-thermal
tails. In this way, the data provided by XSM has
a much broader significance The XSM can trace
the evolution of the flare X-ray spectrum from
birth to death, and thus the whole physical
process during the entire eruption. The XSM
will observe the Sun as a star. Hence, the
final interpretation of the data will be similar
to that obtained from other stars by the modern
large X-ray satellite observatories. Further, the
spectral range of XSM overlaps partly with many
of them (e.g. BeppoSAX, ASCA, XMM-Newton,
AXAF-Chandra), and the spectral resolution is
similar, too. Therefore, it will be feasible for
the first time to make direct comparisons between
the various X-ray emission models of stars. The
comparisons will be based on the XSM observations
on the Sun, and on the observations of other
stars and astronomical objects taken with the
large satellite observatories. Coronal
emission of stars is known to have a fairly
strong connection with the magnetic activity. The
XSM observations on the behaviour of the solar
coronal events together with the information from
other solar science programs will thus enable us
to build a more complete picture of the
connections between different aspects of the
magnetic activity in the Sun, and also in other
stars. Acknowledgements The XSM was
designed and realized in close collaboration
between the Department of Astronomy (University
of Helsinki), Division of X-ray Physics
(Department of Physical Sciences, University of
Helsinki), and Metorex Oy. The research is also a
part of the consortium 'High Energy Astrophysics
and Space Astronomy (HESA), which is financed by
the Academy of Finland and the Finnish Technology
Agency TEKES within the framework of the Finnish
space research program ANTARES. More
Information SMART-1 http//sci.esa.int/home/sm
art-1/ XSM http//www.astro.helsinki.fi/project
s/smart/xsm.html D-CIXS http//sspg1.bnsc.rl.ac
.uk/Share/d-cixs.htm